Geostationary orbit transfers using solar electric propulsion with specific impulse modulation

Maximum-payload transfers to geostationary orbit are computed for solar-electric-propulsion spacecraft that employ specific impulse (or equivalently thrust) modulation. The optimal specific impulse profile is obtained by using a direct optimization method that is based on a calculus of variations approach. Engine models with both constant and variable efficiency are considered, and constant input power is assumed. Numerical simulations show that varying specific impulse for a Hall thruster increases the delivered payload mass when compared to transfers with a fixed specific impulse. When thruster efficiency remains constant, modulating specific impulse increases transportation rate (the payload mass advantage over a chemical-propulsion transfer divided by transfer time) by about 5-6%. However, when a more realistic variable-efficiency thruster model is applied, the transportation rate gain from thrust modulation is diminished by a factor of three. This analysis demonstrates that modulating specific impulse would offer little payload delivery gain for geostationary-orbit-raising missions using realistic Hall-thruster models.